drivers/rtc/rtc-dm355evm.c

   1 /*
   2  * rtc-dm355evm.c - access battery-backed counter in MSP430 firmware
   3  *
   4  * Copyright (c) 2008 by David Brownell
   5  *
   6  * This program is free software; you can redistribute it and/or
   7  * modify it under the terms of the GNU General Public License
   8  * as published by the Free Software Foundation; either version
   9  * 2 of the License, or (at your option) any later version.
  10  */
  11 #include <linux/kernel.h>
  12 #include <linux/init.h>
  13 #include <linux/rtc.h>
  14 #include <linux/platform_device.h>
  15
  16 #include <linux/i2c/dm355evm_msp.h>
  17 #include <linux/module.h>
  18
  19
  20 /*
  21  * The MSP430 firmware on the DM355 EVM uses a watch crystal to feed
  22  * a 1 Hz counter.  When a backup battery is supplied, that makes a
  23  * reasonable RTC for applications where alarms and non-NTP drift
  24  * compensation aren't important.
  25  *
  26  * The only real glitch is the inability to read or write all four
  27  * counter bytes atomically:  the count may increment in the middle
  28  * of an operation, causing trouble when the LSB rolls over.
  29  *
  30  * This driver was tested with firmware revision A4.
  31  */
  32 union evm_time {
  33         u8      bytes[4];
  34         u32     value;
  35 };
  36
  37 static int dm355evm_rtc_read_time(struct device *dev, struct rtc_time *tm)
  38 {
  39         union evm_time  time;
  40         int             status;
  41         int             tries = 0;
  42
  43         do {
  44                 /*
  45                  * Read LSB(0) to MSB(3) bytes.  Defend against the counter
  46                  * rolling over by re-reading until the value is stable,
  47                  * and assuming the four reads take at most a few seconds.
  48                  */
  49                 status = dm355evm_msp_read(DM355EVM_MSP_RTC_0);
  50                 if (status < 0)
  51                         return status;
  52                 if (tries && time.bytes[0] == status)
  53                         break;
  54                 time.bytes[0] = status;
  55
  56                 status = dm355evm_msp_read(DM355EVM_MSP_RTC_1);
  57                 if (status < 0)
  58                         return status;
  59                 if (tries && time.bytes[1] == status)
  60                         break;
  61                 time.bytes[1] = status;
  62
  63                 status = dm355evm_msp_read(DM355EVM_MSP_RTC_2);
  64                 if (status < 0)
  65                         return status;
  66                 if (tries && time.bytes[2] == status)
  67                         break;
  68                 time.bytes[2] = status;
  69
  70                 status = dm355evm_msp_read(DM355EVM_MSP_RTC_3);
  71                 if (status < 0)
  72                         return status;
  73                 if (tries && time.bytes[3] == status)
  74                         break;
  75                 time.bytes[3] = status;
  76
  77         } while (++tries < 5);
  78
  79         dev_dbg(dev, "read timestamp %08x\n", time.value);
  80
  81         rtc_time_to_tm(le32_to_cpu(time.value), tm);
  82         return 0;
  83 }
  84
  85 static int dm355evm_rtc_set_time(struct device *dev, struct rtc_time *tm)
  86 {
  87         union evm_time  time;
  88         unsigned long   value;
  89         int             status;
  90
  91         rtc_tm_to_time(tm, &value);
  92         time.value = cpu_to_le32(value);
  93
  94         dev_dbg(dev, "write timestamp %08x\n", time.value);
  95
  96         /*
  97          * REVISIT handle non-atomic writes ... maybe just retry until
  98          * byte[1] sticks (no rollover)?
  99          */
 100         status = dm355evm_msp_write(time.bytes[0], DM355EVM_MSP_RTC_0);
 101         if (status < 0)
 102                 return status;
 103
 104         status = dm355evm_msp_write(time.bytes[1], DM355EVM_MSP_RTC_1);
 105         if (status < 0)
 106                 return status;
 107
 108         status = dm355evm_msp_write(time.bytes[2], DM355EVM_MSP_RTC_2);
 109         if (status < 0)
 110                 return status;
 111
 112         status = dm355evm_msp_write(time.bytes[3], DM355EVM_MSP_RTC_3);
 113         if (status < 0)
 114                 return status;
 115
 116         return 0;
 117 }
 118
 119 static struct rtc_class_ops dm355evm_rtc_ops = {
 120         .read_time      = dm355evm_rtc_read_time,
 121         .set_time       = dm355evm_rtc_set_time,
 122 };
 123
 124 /*----------------------------------------------------------------------*/
 125
 126 static int dm355evm_rtc_probe(struct platform_device *pdev)
 127 {
 128         struct rtc_device *rtc;
 129
 130         rtc = devm_rtc_device_register(&pdev->dev, pdev->name,
 131                                         &dm355evm_rtc_ops, THIS_MODULE);
 132         if (IS_ERR(rtc)) {
 133                 dev_err(&pdev->dev, "can't register RTC device, err %ld\n",
 134                         PTR_ERR(rtc));
 135                 return PTR_ERR(rtc);
 136         }
 137         platform_set_drvdata(pdev, rtc);
 138
 139         return 0;
 140 }
 141
 142 /*
 143  * I2C is used to talk to the MSP430, but this platform device is
 144  * exposed by an MFD driver that manages I2C communications.
 145  */
 146 static struct platform_driver rtc_dm355evm_driver = {
 147         .probe          = dm355evm_rtc_probe,
 148         .driver         = {
 149                 .name   = "rtc-dm355evm",
 150         },
 151 };
 152
 153 module_platform_driver(rtc_dm355evm_driver);
 154
 155 MODULE_LICENSE("GPL");